This invention relates to an optical module for communication, or in particular, to a filter suitable for an optical communication system of WDM (Wavelength Division Multiplexing) scheme and an optical module using the filter.
In recent years, the internet has increased in speed to such an extent that the access network from each home is also required to be adapted to the high-speed optical communication. As a module for optical communication for access, a transmit/receive module such as the planar waveguide optical module or a combination of CAN (Controller Area Network) modules has been used. In the planar waveguide optical module, one waveguide is arranged at the first end thereof being optically coupled with an optical fiber. This waveguide is separated into at least two waveguides midway. A light-emitting element is mounted at the second end of the first waveguide, while a light-receiving element is mounted at the second end of the second waveguide. A band pass filter is inserted as required midway of these waveguides. This structure makes it possible to produce a transceiver which transmits a signal from the light-emitting element and receives an optical signal from a base station through the light-receiving element.
In a combination of the CAN modules, on the other hand, a fiber is fixed on a housing of a metal or the like, and the optical signal from the fiber is sent to the receiving end through an optical path switching element, while the optical signal from the light-emitting element at the transmitting end is sent to the fiber through the optical path switching element. At the receiving end, a CAN module mounted with a light-receiving element is arranged to receive the optical signal from the base station. At the transmitting end, on the other hand, a CAN module packaged with the light-emitting element is arranged. In both cases, a band pass filter is arranged midway of the optical path as required. By doing so, a transmit/receive module for optical communication can be produced.
Also, in recent years, there have been developed a WDM filter, in which a band pass filter is formed on a surface of one optical part such as a prism and an optical signal transmitted by this band pass filter is separated from the optical signal received from the base station, and a module having the transmit/receive functions integrated in a single CAN module using the WDM filter.
The WDM filter and the optical communication module using it are described, for example, in the following seven references. First, JP-A-2003-232965 discloses as an example of the planar waveguide-type optical module, a structure mounted with a light-receiving element at an end of a planar waveguide and a packaged structure of an amplifier element. JP-A-2006-71739, on the other hand, discloses a structure of a filter arranged midway of a planar waveguide to separate the input/output signals of a laser diode (hereinafter referred to as LD) and a photodiode (hereinafter referred to as PD). JP-A-2005-316291 discloses a module structure for multiplexing optical signals using a WDM filter. JP-A-2007-17903 discloses a transmit/receive module combined with a CAN-packaged light receiving module to transmit the light from the LD and receive the light from the base station through an optical path switching element. JP-A-2004-294513 discloses a transmit/receive module in which the LD and the PD are mounted in a single CAN-packaged module together with an optical filter, and coupled to an optical fiber external to the module through an optical filter. JP-A-2005-157136 discloses a structure in which a band pass filter is formed on a surface of a prism as a WDM filter. JP-A-2005-249966 discloses a structure forming a high isolation degree WDM filter by combining filters of dielectric multilayer films.